Jökull - 01.12.1977, Side 41

A ratlier abrupt increase in the grain size and
the proportion of lithics in the tephra occurs
in the uppermost part of the white unit. Tliis
horizon has been termed (d). Mean grain size
of the pumice is —2.5 $ (5.7 mm) in the profile
at Saudafell and fragments of rhyolite, basalt,
palagonite and ignimbrite make up to 42 weiglit
% of the tephra. The largest fragments are
rhyolite.
Greyish-yellow unit.
This unit, characterized by greyish-yellow
pumice, is reversely gradecl and appears to fol-
low sub-unit (d) without any sudden change in
grain size, which reaches maximum in the upper
part of it. Apparently, the colour shift is not
followed by any sudden changes in eruptive
behaviour. The lithics are of similar amount
and composition as in sub-unit (d). The volume
of the greyish-yellow unit is somewhat less than
that of the white unit. The tephra was mainly
carried towards NE.
The changes in the proportions of the re-
spective components of thc lithic ejecta through
the white unit and the greyish-yellow unit in-
big. 18. Possible weather conditions at the be-
ginning of the eruption that produced H4.
Wholedrawn lines: wind direction at ground
level; broken lines: wind direction at higher
levels.
Mynd 18. Likleg vindátt við jörðu niðri (heil-
ar línur) og hátt í lofti (brotnar línur) i upp-
hafi þess goss er myndaði H4.
dicate changing loci of the explosions, either
along the Hekla fissure or at increasingly deeper
levels.
Greyish-brown unit.
In the thircl unit the pumice is of greyish-
brown colour. The heavy erosion of H4 makes
all estimates of this part difficult. Grain size
is decreasing again and rock fragments are more
scarce, indicating a decline in the force of the
eruption.
Brownish-black unit.
The uppermost unit is characterized by
brownish-black pumice. A rather abrupt cliange
in grain size within this unit may mark the end
of the Plinian phase of the eruption. Black ash-
size ejecta were thrown out for a period of time
after the ceasing of the Plinian phase, sintilar to
the ash accompanying the lava eruption in 1947.
The brownish-black pumice was mainly carried
towards W.
Duration of the Plinian phase.
The detailed ntapping of H4 made it pos-
sible to follow changes in wind direction dur-
ing the l’linian phase of the eruption and thus
estimate its duration.
At the beginning of the eruption wind was
blowing from S or SSE and that wind direction
prevailed during the formation of most of the
white tephra (sub-units (a) to (c)). Then wind
shifted towards SW blowing frorn that direction
while the greyish-yellow and, in part, tlie grey-
ish-brown tephra was erupted. Towards the end
of the Plinian phase wintl shifted again to-
wards SE and E.
This corresponds roughly to passing of one
depression over the country from the southwest,
which may have taken 24—30 liours or so. This
indicates a tephra production averaging 100.000
—80.000 m3/s during the Plinian phase.
THE ACID HEKLA LAYERS IN THE
VICINITY OF THE VOLCANO
The following description of these layers re-
fers to the immediate vicinity of Hekla. The
extension, volurne, and age of these layers are
listed in Table II. Their extension on land is
shown on the maps in Figs. 1—4.
JÖKULL 27. ÁR 39